Don't rely on uninitialized value in MERGE / DELETE

Don't rely on uninitialized value in MERGE / DELETE

On MERGE / WHEN MATCHED DELETE it's not possible to get cross-partition
updates, so we don't initialize cpUpdateRetrySlot; however, the code was
not careful to ignore the value in that case.  Make it do so.

Backpatch to 15.

Reported-by: Alexander Lakhin <[email protected]>
Reviewed-by: Dean Rasheed <[email protected]>
Discussion: https://postgr.es/m/17792-0f89452029662c36@postgresql.org